1. Field of the Invention
The present invention relates to an optical system for use in an image projector (hereafter such an optical system will be referred to as a "projector optical system"), and more particularly to a projector optical system for use in an image projection apparatus (such as a liquid crystal projector) for projecting an image from a reflection-type display panel (such as a reflection-type liquid crystal panel) onto a screen.
2. Description of the Prior Art
As a method for achieving appropriate illumination in a projector optical system of the type that projects the image displayed on a reflection-type display panel onto a screen, U.S. Pat. No. 5,552,938 and Japanese Laid-Open Patent Application No. H5-203872 propose directing the light for illumination to the reflection-type display panel by the use of a polarized-light separating prism disposed in the position of the aperture stop of the projector optical system. FIG. 7 shows the outline of the structure of such a projector optical system. This projector optical system is provided with a projection optical system and an illumination optical system. The projection optical system is composed of a front lens unit (GrF), a polarized-light separating prism (Pr2), an aperture stop (A), and a rear lens unit (GrR). The illumination optical system is composed of a condenser lens (CL).
The light beam from a light source (1) is formed into a substantially parallel beam by a reflector (2), and is then condensed by the condenser lens (CL) so as to form an image of the light source. The light source (1), the reflector (2), the condenser lens (CL), and the polarized-light separating prism (Pr2) are so arranged that the image of the light source is formed in the position of the aperture stop (A). Thus, this structure conforms to that of the so-called Koehler illumination. Of the light beam that is condensed to form the image of the light source, only the S-polarized light component is reflected by the polarized-light separating prism (Pr2). The light beam reflected from the polarized-light separating prism (Pr2) then passes through the rear lens unit (GrR), and then enters a color separating/integrating prism (Pr1), where the light beam is separated into three light beams of different colors so as to illuminate the display surfaces of three reflection-type display panels (PR, PG, and PB) individually, with each light beam illuminating the entire display surface of the corresponding display panel.
Since these display panels (PR, PG, and PB) employ reflection-type liquid crystal panels, the light beam that illuminates each of the display panels (PR, PG, and PB) is, when reflected therefrom, partially P-polarized and partially S-polarized according to the pattern formed by the pixels of the display panel. The light beams reflected from the individual display panels are then, by the color separating/integrating prism (Pr1), integrated into a single light beam to be projected (hereafter referred to as the "projection light beam"), which then passes through the rear lens unit (GrR). Thereafter, of this projection light beam, only the P-polarized light component is allowed to pass through the polarized-light separating prism (Pr2). Here, note that the front lens unit (GrF) is designed to be substantially afocal so that the rays from around the center of each of the display panels (PR, PG, and PB) pass through the polarized-light separating prism (Pr2) as a nearly parallel beam. After passing through the polarized-light separating prism (Pr2), the projection light beam passes through the front lens unit (GrF), and then forms a display image on a screen (S).
In a case where a projector optical system of the type that projects the image displayed on a reflection-type display panel onto a screen is employed in a projection television system of a backward-projection type (rear type), a wide angle of view needs to be achieved in the projection optical system provided in the projector optical system. In the above-described conventional example, in order to achieve a wide angle of view in its projection optical system, it is essential to increase the angular magnification offered by its front lens unit (GrF). However, if the angular magnification is increased, the front lens unit (GrF) will have an unduly large diameter. This is because the front lens unit (GrF) is designed to be afocal, i.e. to have almost zero optical power.
Moreover, in a case where a high-efficiency light source such as a metal halide lamp is employed in order to secure sufficiently bright illumination in the projector optical system, an integrator is additionally required therein to prevent uneven illumination that such a light source tends to cause. In the projector optical system, like the above-described conventional example, that is so designed that the light source image is formed in the vicinity of the aperture stop (A) of its projection optical system, the use of an integrator necessitates the use of an illumination relay optical system to allow the light source image formed within the integrator to be re-focused in the vicinity of the aperture stop (A) of the projection optical system. In this case, however, since the front lens unit (GrF) of the projection optical system is designed to be afocal, the illumination relay optical system needs to be designed as a telecentric optical system. To be telecentric, however, the illumination relay optical system needs to be unduly large.